Ever get a word in your mind that won't go away, but which you can't remember the meaning of?

I had that happen the other day, and it nearly drove me crazy till I finally looked it up.

The word I had in mind is "phenology," which has to do with observing the natural world and keeping a record of it. Specifically, it is that branch of science dealing with the relations between climate and periodic biological phenomena, such as bird migration or plant flowering.

The Swedish botanist Linnaeus (1707-1778) is considered the father of modern phenology. During his lifetime he established a network of 18 observation stations in Sweden, and in the process shed light on the patterns of interdependency of all things.

Thanks to the efforts of thousands of phenologists, amateur as well as professional, we know without doubt that most natural events in the annual cycle recur in regular order on or near the same date for any certain location. We've also learned a great deal about the limits within which these events occur.

According to information presented in Jim Gilbert's book "Minnesota Nature Notes," for example, the mean annual temperature across our state ranges from 36 degrees F in the north-central and northeastern counties to a bit over 46 degrees in the extreme southeast. Record temperatures have been as high as 114 (July 6, 1936, in Moorhead) and as low as 60 below (Feb. 2, 1996, at Tower), a 174-degree range that is unrivaled over most of the planet.

Because temperatures vary from place to place and month to month, almost all recurring natural events fall within a band of dates, rather than on a specific day each year. But taken broadly, and considering the differences in available sunlight from north to south, each month brings with it some special occurrences.

January, on average, is the coldest month of the year in Minnesota, yet fleeting signs of spring are already in the air, most noticeably the songs of white-breasted nuthatches, northern cardinals and black-capped chickadees. We almost always have a warm period - the "January thaw," which was noticeably absent this winter - marked by melting temps, sunshine and south to westerly winds.

February often brings skunks and raccoons out of their winter dens, though thousands of ice-fishing houses still dot our lakes. March is usually a mixture of warm and cold, often the peak of the snow season but also our first month of real spring. Canada geese are flying and honking, maple sap begins to flow, the first returning robins arrive, and if the temperature on the snow tops 27 degrees, snow fleas abound.

With April comes wind and rain and a greening landscape. The ice-out of lakes begins, though once in a great while some lakes are ice-bound into May. Rhubarb begs to be harvested, crabapple trees will soon begin to bloom, and our state bird - the common loon - returns from points south.

In May the greening process hits full stride. Rose-breasted grosbeaks and orioles arrive at feeding stations. Lilacs bloom. White-tailed deer fawns are born. Corn, soybeans, spring wheat, potatoes and other farm crops are planted, and the Monarch butterflies return.

June can be the month of rains here in Minnesota, yet frost is always possible in low areas and in the north.

But you get the picture. Most of the changes in nature occur on a repetitive basis. We've seen them before, and we'll see them again. Which, when you think about it, is only natural - and vastly reassuring.